The invention concerns a resonator system for generating a radio frequency (RF) magnetic field in the investigational volume of a magnetic resonance (MR) arrangement, comprising a number N of individual resonators which surround the volume under investigation and which are each disposed on flat, dielectric substrates about a z-axis, wherein the individual resonators have windows through each of which one individual RF field is generated in the volume under investigation during single operation of the individual resonators, and, through cooperation among the individual resonators, a useful RF field is generated in the volume under investigation, wherein a remote RF field is asymptotically generated far outside of the resonator system, the spatial distribution of the useful RF field being substantially mirror-symmetrical with respect to a first plane A which contains the z axis, and that of the asymptotic remote RF field is substantially mirror-symmetrical with respect to a second plane B which contains the z-axis, wherein the useful field extends substantially parallel to the first plane in the volume under investigation, and the number of individual resonators is N>4, wherein, during operation of the resonator system at at least one point in time, the substrate plane of at least one individual resonator subtends an angle of more than 40° with respect to the direction of the useful RF field in the volume under investigation, and at least one further individual resonator subtends an angle of less than 40° with respect to the direction of the useful RF field in the volume under investigation.
A resonator system of this type is disclosed in reference [9].
NMR is a very distinctive and exact method for structural analysis of chemical compounds. RF resonator systems are used to irradiate the RF pulses into the sample and to also detect radio frequency electromagnetic fields from the sample. An RF resonator system consists of one or more RF resonators which are disposed in the direct vicinity of the sample for the NMR measurement. The use of cooled and, in particular, superconducting radio frequency resonators minimizes the losses in the resonator and thereby considerably increases the sensitivity to obtain a maximum receiving sensitivity, i.e. a maximum S/N ratio.
HTS materials are currently best suited as superconductors. They have a high transition temperature and are very insensitive to static magnetic fields compared to other superconductors. These positive properties, in particular, the high critical current densities are obtained only if the superconductor consists of very thin epitactic layers which are formed on oriented single-crystal substrates, wherein these substrates are usually available only as flat plates. The geometric shape of the individual resonators cannot therefore be arbitrarily selected but must be formed from flat plates. This strongly limits the possible geometric designs.
Arrangements of this type are disclosed e.g. in references [1] to [9]. A substantial problem involved with the use of superconducting receiving resonators is the static magnetization generated by the superconductor which can produce field inhomogeneities and strongly inhibit obtaining narrow lines in high-resolution NMR. Moreover, the limited critical current in the superconductor limits the maximum coil current of the transmitting pulse and thereby complicates or prevents short pulse widths for a given NMR flip angle.
Resonator systems are conventionally operated in quadrature to effectively excite the spin system. This is only possible with resonator systems having several resonators. Towards this end, spatially rotating fields are generated, i.e. the second resonator is excited with transmitting pulses whose radio frequency phase is shifted by 90° relative to the transmitting pulses of the first resonator. The spatially rotating field thereby generated is much more efficient in exciting the spin system compared to the normal spatially stationary field. The quadrature method also produces a factor √2 of the S/N ration compared to the conventional method with only one resonator, since the noise of the two resonators is not correlated. The maximum B1 excitation field (useful RF field) generated by the resonator system also increases by the factor √2. This method is particularly suited to measure large-loss samples with a high S/N ratio, e.g. salt solutions.
Irrespective of the mode of operation of the resonator system, the system should produce a B1 field with optimum homogeneity and permit a second resonator to be mounted as closely as possible to the sample.
Reference [10] discloses a hybrid Birdcage resonator which is constructed from solid or slotted superconducting strip conductors 101 which are optimally disposed around the sample, parallel to the axis of the sample (FIG. 13). These strip conductors 101 are electrically interconnected via a capacitor formed at each end with a normally conducting tubular element 102. Since the strip conductors 101 are disposed very close to the sample, have a practically pure inductive effect, and must not fulfil a capacitive function, they can carry the full critical current. Due to these two reasons, the maximum B1 field which can be generated in the volume under investigation obtains an optimum value and is homogeneous. Accordingly, this resonator type also permits optimum short pulse widths for a desired NMR flip angle. The highly compact construction offers optimum installation possibilities in particular for samples with large diameters which largely limit the remaining space. For small sample diameters, the somewhat complicated structure limits the geometry. These resonators are also particularly suited for operation in quadrature mode and are very efficient. The Q factor is, however, relatively small due to use of normally conducting material for the tubular elements 102. Moreover, the construction of hybrid birdcage resonators is quite demanding.
Reference [11] discloses a quadrature-suitable Birdcage resonator which is built from superconducting material only and whose bars 103 which extend parallel to the sample axis contain meandering conducting structures 104 (FIG. 14). Since the overall resonator is constructed from superconducting material, its quality factor is high. The meandering structure of the resonator additionally generates undesired electric fields which have a negative influence on the losses in conducting samples. Moreover, this arrangement has a very poor field homogeneity in the z direction, which reduces the S/N ratio, and a very low current carrying capacity, which precludes short pulse angles. The same applies if the meandering bars are not operated in self-resonant mode but are capacitively coupled via the normally conducting ring.
Reference [11] also discloses a resonator system made from a plurality of individual elements 105 which are radially symmetrically disposed about a z-axis in the form of a star (FIG. 15a). The individual elements 105 each consist of two parallel HTS layers disposed on a substrate, forming a capacitance 106. The individual substrate plates are brought sufficiently close together during assembly, such that a further capacitance 107 is formed between the HTS layers of neighboring individual resonators 105. In order to be able to neglect the additional capacitances 107, such that the resonance frequency is only provided by the capacitances 106 defined between the electrodes, the separations would have to be in the micrometer range. However, this is technically not possible. FIG. 15b shows an equivalent diagram of such a resonator system. The capacitor drawn on a large scale represents the capacitance 107 between neighboring individual resonators 105 and is thereby given by the air/vacuum gap 108. The small capacitor represents the capacitance 106 between the electrodes of a substrate. The dots in FIG. 15b symbolize more bars with corresponding capacitors mounted to both sides. The structure is closed, i.e. the left-hand edge is capacitively connected to the right-hand edge. When the resonator system resonates, current can flow from one individual element to the next through the capacitors at the ends of the individual elements. The current in the “loops” at the top and bottom is defined as an effective displacement current in the capacitors 106, 107. Combination of the capacitors 106, 107 produces the equivalent diagram of FIG. 15c, illustrating a conventional high-pass Birdcage resonator. The field distribution of this arrangement in the xy plane is shown in FIG. 15d. 
References [1]–[8] disclose Helmholtz resonators which have a very high quality factor due to the exclusive use of HTS material. With appropriate geometrical arrangement (positioning two conductors 109 at an angle of 120° as shown in FIG. 16), this arrangement can obtain relatively good field homogeneity. The efficiency is however small, since the current carrying elements are located remote from the sample. Moreover, the conventional arrangements require a large amount of space. For this reason, resonators for further frequencies are located considerably further away from the sample or cannot be mounted at all due to lack of space. Another disadvantage is that these devices are quadrature-suitable to only a limited degree, namely only if the conductors 109 are disposed at an angle of less than 90°.
Reference [9] discloses “Twin-V” resonators which are composed of several identical individual resonators 111 mounted onto planar dielectric substrates 110 (FIG. 17). With a given current through the resonance circuit, this device generates an optimally large B1 field at the center of the volume under investigation 112, wherein the direction of the B1 field extends approximately parallel to the plane of the substrate 110 and the parallel displacement should not exceed 40°. In addition to the high quality factor and the good field homogeneity of the above-described resonators, a relatively good efficiency is realized. Disadvantageously, this arrangement can only be used for small samples and generates a large stray field which produces strong coupling with other resonators. Moreover, in the embodiment shown in FIG. 17, the arrangement can only be operated in a linearly polarized mode.
Reference [9] also discloses use of arrangements with components 113 comprising several individual resonators to also generate spatially rotating fields. An arrangement of this type is shown in FIG. 18a. The components 113 are disposed around an axis in the shape of a wind wheel. FIG. 18b shows the magnetic field 114 (individual RF field) generated by one individual resonator. Only part of the individual RF fields of the individual resonator, in the present case the field 114 generated by the current (+), is used to generate the useful RF field in the volume under investigation 112. The field 115 generated by the counter current (−) on the other side of the individual resonator is disposed as far away from the sample as possible to prevent cancellation or unnecessary disturbance of the useful RF field. The rotating field can be generated more efficiently by using n components 113 which are designed as shown in FIG. 18a and which are symmetrically disposed about the volume under investigation 112. A component 113 is disposed such that the conductor 116 lies perpendicularly over the center of the volume under investigation 112. Subsequent to the first component 113, n−1 further identical components 113 are added, wherein these are rotated about the longitudinal axis of the volume under investigation 112 by 360°/n, relative to the first. A rose like, symmetrical structure is obtained for exciting a rotating field. FIG. 18c shows an arrangement of this type comprising eight individual resonators disposed on components 113. The remote RF field of such resonator systems has a symmetry plane B which is rotated relative to the symmetry plane A of the useful RF field by an angle 360°/n, i.e. by 45° in the example of FIG. 18c. If more individual resonators are used, the angle becomes smaller and smaller. N>8−10 can hardly be realized for high-resolution NMR coils due to the geometry (as explained in reference [9]), such that, in practice, the angle of the remote field is rotated relative to the useful RF field by more than 36°. A result of this rotation of the useful RF field relative to the remote RF field is that the mounting and, in particular, inductive coupling of further resonators for additional frequencies in such a manner that the coupling between the resonator system and the further resonators can be neglected, is extremely difficult. It should also be noted that the resonator system contains a field (individual RF field) which is considerably higher than the useful RF field. This embodiment requires even more space than an arrangement with Helmholtz pairs such that the resonators for further frequencies are located at a considerably greater distance from the sample or cannot be mounted at all due to lack of space.
As has been shown, all conventional resonator types have some serious disadvantages.
It is the object of the present invention to further develop the resonator system of [9] to realize high efficiency, full quadrature suitability and good field homogeneity with a simple and compact structure.